In many industrial processes mixtures of air and condensable vaporous materials such as gasoline, halogenated hydrocarbon solvents and the like are produced. It is ordinarily economically and ecologically desirable to recover the condensable vaporous materials for disposal or for reuse.
There are a number of different approaches used commercially to collect and recover these vapors. One of the preferred approaches involves the use of activated carbon adsorber beds. Plural beds are used so that one bed may be regenerated while the other bed is on-stream performing its adsorption function.
The usual adsorption system of this type comprises two or more adsorber vessels containing an activated carbon adsorbent. The adsorbable gas-laden air to be treated is passed through a bed of adsorbent. The adsorbable components are adsorbed and the purified air is vented to the atmosphere. When the adsorbent has adsorbed substantial adsorbable components, the influx of the adsorbable component-laden air is terminated and the adsorbed component is driven off by heating (temperature swing) and/or applying a vacuum (pressure swing) to the adsorbent bed. A purge gas may be bled through the bed being desorbed. The purge gas/adsorbable component mixture is then separated by a second technique such as condensation and the remaining adsorbable component contaminated purge gas is recycled to a bed in the adsorption step. The condensed adsorbable gas is then recovered.
U.S. Pat. No. 4,305,734, which is incorporated herein by reference, discloses a system wherein a hydrocarbon, e.g. methane, is adsorbed onto an activated carbon. Methane is used to displace the air in the bed that is being desorbed so that the hydrocarbon recovered will not be contaminated with air when desorbed. Just enough methane is used to displace one actual void volume of the adsorbent bed. An "actual void volume" is the amount of a gas required to just displace the gas in the adsorbent bed at the actual pressure at which the bed happend to be.
U.S. Pat. Nos. 4,066,423; 4,261,716; 4,276,058; and 4,462,811, all of which are incorporated herein by reference, disclose a system wherein the gas mixture from the desorption step is passed through a liquid absorber and then through a second adsorber to remove traces of remaining adsorbable components. Improvements cited in some of these patents include bleeding of the non-adsorbable gas (air) through the desorbing bed, use of a liquid ring vacuum pump, manipulation of the circulation rate in the absorber to maximize absorption and so on. A disadvantage of these processes is that a second type of apparatus with its own separate and distinct problems is used.